1. Field of Invention
The present invention relates to the field of semiconductor processing. More specifically, the invention is a method of plasma etching through dielectric material to form holes for interconnecting a lower layer of metal with an upper layer of metal through structures opened within the dielectric material known as vias.
2. Background
In semiconductor integrated circuit device fabrication, an aluminum layer is often deposited and then may be etched to create a set of wires or interconnects 12 (FIG. 1), over which is deposited a layer of dielectric or insulating material 10, often a silicon dioxide. The dielectric layer 10 is masked and etched to open narrow profile openings known as vias (shown in phantom at 11 in FIG. 1). Each via respectively extends as an opening through dielectric layer 10 to a portion of the lower aluminum layer or to a wire 12. Another layer of metal 14 is deposited over the dielectric film. As shown in FIG. 2, the deposited metal fills the vias, forming metallic contact structures 13 engaging the exposed underlying metal at the bottom of the vias and making points of contact through layer 11 between the lower wires 12 and the upper metal layer 14 (which may also in turn be etched to create a layer of wires). The geometries of the vias 11 and the contact structures 13 which now fill them are usually circular although they may also form a trench shape. Note that vias 11 have been positioned so that the metallic structures which fill them provide contacts between two separated wire or metal layers; such vias which perform this function are also termed "contact vias" or simply "vias".
To open the via structures, the dielectric film is etched in a plasma processing reactor. Fluorine-based gases, such as CHF.sub.3, CF.sub.4, C.sub.2 F.sub.6, sometimes in combination with O.sub.2, Ar or He are typically the most effective plasmas for obtaining desired etch rates, selectivities and uniformities. A typical process for etching a dielectric layer to form a via is described in Arleo et al, U.S. Pat. No. 5,176,790, incorporated herein by reference, which discusses etching the dielectric layer in a plasma of a mixture of fluorine-containing gases and nitrogen-containing gases.
A problem encountered in etching a via is the deposition of a residue 16 onto the sidewall of a via. An example of such a residue is shown in FIG. 3. It is thought that the residue is a mixture of organic material and metal, with the organic material contributed by the processing gases and sputtered photoresist, and the metal portion from sputtered contributions from the metal layer at the bottom of the via. The residue forms during the etching process as the via is opened through layer 10, thus exposing the underlying metal to the plasma. For this reason the residue is referred to as backsputtered metal. The backsputtered metal residue has been a perennial problem in manufacturing integrated circuit devices because it alters the resistivity of the subsequent metal-to-metal connection through the vias, which can lead to device failure.
In the more modern device structures the metal backsputtering problem is exacerbated. One reason is the common use of topographic metal layers at different depths underlying the dielectric film together with the planarization of the dielectric film on top of the metal. Such planarization of the dielectric film is necessary for more precise masking of the dielectric film with via shapes and, in turn, more precise etching of the dielectric to form the vias, but it results in thinner dielectric film areas where the underlying metal is closer to the surface of the planarized dielectric layer, and thicker dielectric film areas where the underlying metal lies more deeply underneath the dielectric. Then during via etching of the planarized dielectric layer positioned over topographical metal, the shallower areas of the planarized dielectric are opened to the underlying metal before the thicker areas. The metal at the bottom of the shallower vias is thereby exposed to plasma during the overetch period, that is, the additional time over which the process continues to complete the etching of the deeper vias. Because of the varying depths of the vias, the overetch period may have to be relatively prolonged. It is during the time in which metal at the bottom of the via is exposed to plasma that the metal sputters onto the sidewalls to form unwanted residue.
The residue is difficult, if not impossible to remove in a subsequent plasma photoresist stripping process. The residue is not a pure metal, but rather a mixture of metal and organic etch residues. The photoresist stripping process may remove some of the organic components of the residues, but the backsputtered metal portions and compositions thereof remain. The remaining residue must be removed, however, or else either the resistivity will be altered for the subsequent interconnection metallization (i.e., the metallic contact structures 13) or particularly in higher density devices, the via may become so clogged with residue that the formation of the interconnection metallization is impeded. An additional wet chemical treatment is conventionally used to remove the remaining residue. A problem with wet chemical treatment, however, is that it is an additional processing step. Wet chemical treatment adds undesirable cost to manufacturing production. A better way to deal with the backsputtered metal residue problem is to remove it before photoresist stripping or prevent it from occurring. Prevention was the attempt of the process disclosed in U.S. Pat. No. 5,176,790 by using process gases that heavily deposited polymer material within the vias during etching. However, for more highly topographic metal as is typically found in modern semiconductor integrated circuit devices, a lengthy overetch is required during the dielectric etch. If the process of U.S. Pat. No. 5,176,790 is used for a long overetch an excessive amount of polymerization occurs, which in turn is difficult to remove completely. Hence, the process disclosed in U.S. Pat. No. 5,176,790 is not sufficient for modern semiconductor integrated circuit structures.
Although presented in a different context, the concept of adding a small amount of gas to remove unwanted polymer residue remaining following a silicon dioxide over silicon contact etch was explored in Abstract No. 303 by Okumura et al, in the Electrochemical Society Conference of 1980. Okumura et al added chlorine to the silicon dioxide contact etch process, and found that the chlorine helped to dissociate C-F bonds in the silicon dioxide etch process chemistry and removed polymer residue at the surface of silicon exposed upon etching through the silicon dioxide layer. But in the context of via etching, that is, etching a dielectric and stopping the process upon reaching an underlying metal film, it is not sufficient to add a gas that works simply by enhancing dissociation of C-F bonds for removing metal residue.